The Hackaday Superconference is all about showcasing the hardware heroics of the Hackaday community. We also have a peer-reviewed journal with the same goal, and for the 2018 Hackaday Superconference we got a taste of the first paper to make it into our fully Open Access Journal. It comes from Ted Yapo, it is indeed a tale of hardware heroics: what happens when you don’t want to spend sixty thousand dollars on a vector network analyzer?
Ted’s talk begins with a need for a network analyzer. These allow for RF measurements, but if you ever need one, be prepared: you can spend twenty thousand dollars on a used VNA. Around the time Ted’s project began, Rigol released their cheap spectrum analyzer, the DSA815. This thing only cost a thousand dollars. It was their first revision of the hardware, and it was only a scalar network analyzer. Being the first revision of the hardware, there were a few problems; there was leakage that would affect the measurement. The noise floor was higher than it should have been. These problems can be corrected, though, with a little bit of cunning from Ted:
Ted’s Rigol spectrum analyzer consists of two basic parts. The tracking generator generates a signal, which is fed through the device under test. The spectrum analyzer receives the signal coming from the tracking generator, through the device under test, and puts lines and numbers on the screen. If you’re measuring how well a device performs at 100 kHz or 1 GHz, this is what you need.
The problem comes with the leakage between the tracking generator and the spectrum analyzer. Without anything plugged in, the noise floor of this spectrum analyzer was at -90dB, while the leakage was measured at around -70dB. This was measured through an HP355D variable attenuator, and for measuring signals that low, this cheap spectrum analyzer was basically useless.
This was a scalar network analyzer, though, not a vector analyzer. Ted found the effect of the leakage on measurement results could be manipulated by changing the phase shift of the signal. Simply by using a commercial diode mixer (a few diodes and a few transformers), the phase of the signal from the tracking generator can be inverted 180°. Since the phase is inverted, the measured signal changes with respect to what it should be. A little bit of math, and you can reasonably correct for this leakage in the machine.
The result with this new setup is a scalar network analyzer that’s remarkably better than the stock machine — up to 30dB of dynamic range better. Keep in mind this problem has been corrected in newer revisions of the Rigol spectrum analyzer (Ted says the serial number of his machine is very low, in any event), and this shouldn’t be taken as a review of this particular network analyzer. It is, though, a remarkable tale of hardware heroics and using brainpower when you don’t have the cash to buy the fantastically expensive tools you need.
Ted’s talk is great, and he’s also written a paper on his work, published in the first edition of the Hackaday Journal of What You Don’t Know. We’re always looking for new articles in our Open Access Journal, so if you have a sordid tale of doing something that shouldn’t be possible, think about submitting your own paper to the journal.